This invention relates generally to fiber optic gyros and more specifically to fiber optic gyro sensor coils and the optical fiber with which they are wound.
Fiber optic gyros sense rotation by measuring the phase difference in light waves that propagate in opposite directions through a coil wound with optical fiber. Light waves that propagate through the coil in the direction of rotation take a longer time than light waves that propagate through the coil in the direction opposite to the direction of rotation. This difference in time, measured as the phase difference between counter propagating light waves, is proportional to the angular velocity of the coil.
The phase difference between counter propagating light waves in a fiber optic gyro sensor coil is affected not only by the angular velocity of the coil but also by changes in the physical properties of the coil brought about by changes in environmental parameters, temperature being the most important. A change in temperature of the coil changes the index of refraction and the physical dimensions of the optical fiber thereby directly affecting the time it takes for a light wave to propagate through the coil.
If the rate of change of temperature were low enough that the coil had substantially the same temperature throughout its volume, the changes in propagation times of the counter propagating waves would be substantially the same and the measured phase difference would be substantially independent of the rate of change of temperature. However, at rates of change of temperature that fiber optic gyros encounter, the counter propagating waves experience different temperatures at a given time and complete cancellation of the temperature effects do not occur. As a result the phase difference as measured by a fiber optic gyro includes a bias which must be subtracted from the measured phase difference to obtain the phase difference attributable to the rotation of the coil.
It is possible to accurately model the bias of coils designed for high-accuracy applications with a function of temperature (the "modeling function") which is related to thermally-induced nonreciprocities. (See D. M Shupe, Thermally Induced Non-Reciprocity in the Fiber Optic Interferometer, 19 Applied Physics 654 (1980). See also U.S. Pat. No. 5,371,593, Sensor Coil for Low-Bias Fiber Optic Gyroscope, A. Cordova et al.) In order to further improve the accuracy of these gyros, it is very important to eliminate, or at least to reduce substantially, the temperature dependence of the coefficients that define the modeling function.